SIL

IEC 61508 defines four safety integrity levels, SIL 1 through SIL 4, each associated with a target range for probability of failure on demand or per hour, depending on whether the safety function operates in low-demand or high-demand/continuous mode. The level applies to a safety function, not to a whole system or product, so a single piece of equipment can implement functions at different SILs.

How SIL is determined and used

• Risk assessment establishes the required risk reduction for a hazard, which is then expressed as a target SIL for the safety function that provides it.
• The target SIL sets architectural constraints, such as hardware fault tolerance and diagnostic coverage requirements.
• It also sets process rigour: the techniques required for design, verification and functional safety assessment scale with the level.
• Sector-specific standards, such as IEC 61511 for process industries or IEC 62061 for machinery, derive their own SIL frameworks from IEC 61508.

A common confusion is treating SIL as a property of a product rather than of a specific safety function within a specific application. The same device certified for use in one SIL context may not automatically be suitable at the same level in a different application, because the required risk reduction depends on the application's own hazard analysis.